ISRO's PSLV-C29 rocket launches six Singaporean satellites

Sriharikota (Andhra Pradesh): India on Wednesday successfully launched six Singaporean satellites. As the Indian space agency achieved the milestone of its 50th rocket launch from here, its chief said they are now looking at the way forward and have a lot more to achieve.

The Polar Satellite Launch Vehicle's core alone (PSLV-CA) variant -- standing 44.4 metres tall and weighing around 227 tonnes -- took off from the Sriharikota rocket port in Andhra Pradesh, around 80 km from Chennai. It rose into the evening skies, riding majestically on the tail of fierce orange flames.

Expressing satisfaction at ISRO's performance in 2015, the space agency's chairman A.S. Kiran Kumar said it was now way forward to 2016.

ISRO will carry on the activity of putting satellites -- both communication and earth observation variants -- into orbit, he said.

"We have a lot more to achieve next year," he said.

Another official said with Wednesday's successful blast-off, ISRO also achieved the milestone of its 50th rocket launch out of Sriharikota.

Besides launching the six Singaporean satellites, ISRO also tested the rocket's fourth stage/engine's ability to restart after it was cut off around 17 minutes into the flight.

Technically speaking, India was testing a multiple burn fuel stage/rocket engine for the first time.

"The restart and shut off of the fourth stage engine is done as a first step towards launching multiple satellites but in different orbits," an ISRO official told IANS, declining to be named.

Launching multiple satellites with a single rocket is nothing new for ISRO and it has been doing that for several years now. The challenge is, however, to put several satellites into different orbits with one rocket.

This is what ISRO plans to test when the PSLV-CA ejects the six Singaporean satellites.

The PSLV rocket is a four-stage/engine rocket powered by solid and liquid fuel alternatively.

"Restarting a rocket engine soon after it is shut off is a critical technology that has to be mastered. Once a rocket engine is activated, then the heat generated is very high. The trick is to cool it down in space and to restart it after a short gap," an industry expert told IANS.

"This is entirely different from switching on and off the communication satellite's engines in space. The interval between two restarts of a communication satellite engine will be in days. But in the case of restarting a rocket engine, the time gap will be in hours," the expert added.

"By that time the rocket's engine has to be cooled down. This part of the experiment is very critical," he explained.

The PSLV's fourth stage/engine will be restarted just over 67 minutes into the flight or 50 minutes after the engine was cut off.

At the time of restart, the fourth stage will be in a lower altitude of 523.9 km while the satellites would have been ejected at 550 km altitude.

The engine will be operated for four seconds and is planned to go up to an altitude of 524 km before it is cut off again.

In December 16 blast-off, ISRO is using the 'core alone' variant of the PSLV rocket. The rocket did not have the strap-on boosters, its standard feature.

With the successful launch of the six Singaporean satellites, ISRO has put a total of 57 foreign satellites into space.

Out of the six satellites, the 400 kg earth observation satellite called TeLEOS-1 is the main passenger for the PSLV rocket and hence the mission is called TeLEOS mission by ISRO.

TeLEOS-1 is Singapore's first commercial earth observation satellite designed and developed by ST Electronics.

So far in 2015, ISRO has launched 14 satellites (three Indian and 11 foreign) from its rocket port in Sriharikota. Thirteen satellites were launched with PSLV rocket and one communication satellite -- GSAT-6 -- with geosynchronous satellite launch vehicle (GSLV).

Last month, India also launched its communication satellite GSAT-15 using the Ariane rocket of the European space agency which takes the total number of satellite launches in 2015 to 21 (17 foreign, four Indian).